Clusters as models of bulk surfaces and novel materials

Author

Jin, Changming

Date

1992

Advisor

Smalley, Richard E.

Degree

Doctor of Philosophy

Abstract

Studies on gas phase clusters show that common ground exists between clusters and bulk surfaces such that clusters can serve as models of bulk surfaces. Several cluster systems are studied to probe this cluster-surface analogy with both ultraviolet photoelectron spectroscopy (UPS) and Fourier transform ion cyclotron resonance (FT-ICR). Ultraviolet photoelectron spectra of GaAs cluster anions are measured with a photon energy of 7.9 eV. The electron affinity displays a strong even/odd oscillation suggesting the presence of a substantial gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the corresponding neutral clusters. The absence of unpaired electrons in even numbered neutral clusters is reminiscent of the healed dangling bonds caused by the reconstruction and relaxation at GaAs bulk surfaces.
Chemical reactions of noble and transition metal clusters with CO and H$\sb2$ are studied with FT-ICR. The chemistry and physics of these metal cluster plus adsorbates systems are also modeled well by using the cluster shell model originally developed for metal bulk solids and surfaces. Excellent interplay between theory and experiment on the Cu$\sb{\rm x}\sp+$CO systems reveals the major advantage for the cluster model of bulk surfaces.
New materials are prepared in the course of studying gas phase carbon clusters. Laser vaporization of a graphite/BN composite disk produces fullerenes in which one or more atoms of the hollow carbon cage is replaced by a boron atom. These boron-doped fullerenes are found to act as Lewis acid when they chemisorb ammonia molecules. Fullerenes with one or more metal atoms inside are also generated both in gas phase and in macroscopic quantities. The formation mechanism of the multi-metal doped fullerenes is proposed as due to coalescence. These new materials are expected to have interesting, novel, and useful properties.